1. Field of the Invention
The invention relates to a safety valve used in subterranean wells, such as oil or gas wells, and specifically to valves which are actuated in response to changes in a control fluid pressure applied by one or more control lines extending from the subsurface valve location to the well surface.
2. Description of the Prior Art
Subsurface safety valves are commonly used in oil or gas wells to prevent the escape of fluids from a producing formation in the event of damage to the well conduits or to the surface elements of the well. Typically such safety valves are incorporated into the production fluid transmission tubing which is inserted through the well casing and extends from the surface of the well to the producing formation. The flow of fluids through this inner tubing string must be interrupted in the event of damage to the upper portions of the casing, the tubing string or to the well head. By positioning these valves at a location below the well surface, for example, below the mudline in an offshore well, the safety valve can be closed to prevent the escape of produced fluids.
The most common subsurface safety valves employ either a pivoted flapper or a rotatable ball valve head to open and close the transmission conduit. Commonly, the valve head is actuated from a closed to an open position in response to axial movement of an actuator sleeve.
In recent years, subsurface safety valves have utilized small diameter, axially shiftable pistons or cylinders for moving the actuator sleeve. Such movable pistons or cylinders are disposed in an annular space intermediate the exterior of a central production flow conduit and the outer wall of a valve housing which is secured in series relationship in the production tubing string. Prior art constructions are known wherein the cylinders for such pistons are fixedly mounted in the valve housing, or alternatively, the spindle or piston rods are fixedly secured to the valve housing and the cooperating cylinders are movable relative to the fixed piston rods. U.S. Pat. Nos. 4,005,751, 4,119,146, and 4,161,219 each disclose subsurface safety valves having a spool or piston mounted in the valve housing and operatively connected to the valve actuator sleeve. U.S. Pat. No. 4,503,913 discloses a subsurface safety valve actuator having a piston rod secured to the valve housing and a cooperating cylinder operatively connected to the valve actuator sleeve.
Occasionally, a defect or leak in the small cylinder or piston will prevent the development of sufficient force to shift the valve actuator sleeve to a valve opening position. In such case, the availability of a backup hydraulic system, including a separate hydraulic line extending to the well surface is a desirable adjunct. More importantly, if the primary piston or cylinder is not operable, it becomes desirable that the safety valve be shifted to, and locked in, an open position to insure that wireline tools may be inserted through the safety valve to effect the control of other tools located downhole relative to the safety valve. An apparatus for effecting this backup actuation of the safety valve and/or the locking of the safety valve in a fully open position is disclosed in U.S. Pat. No. 4,796,705.
One disadvantage of the construction shown in U.S. Pat. No. 4,796,705 is the fact that the primary actuating cylinder remains engaged with the actuating sleeve during the movement of the sleeve to its fully open position by a movable secondary cylinder. Thus, additional fluid pressure must be applied to the secondary cylinder to counteract the upwardly directed force of the well fluids acting on the lower surface of the primary cylinder. There is a need, therefore, for an improved apparatus utilizing primary and secondary cylinders to actuate a downhole safety valve to reduce the amount of hydraulic pressure required to operate the secondary cylinder to effect the downward displacement of the valve actuating sleeve to its valve open position, and the locking of the mechanism in such position.